A NAND flash memory includes a sense amplifier which reads data held by memory cells. The sense amplifier includes a detector connected to bit lines. The detector detects data depending on the potential of a bit line.
Specifically, the detector detects two-level data which is, for example, held by a memory cell. The memory cell holds “0” data when charge is accumulated in a charge accumulation layer. When the charge exits the charge accumulation layer, a threshold distribution is positioned on a negative side. This corresponds to “1” data, that is, an erased state.
According to one read method, when a threshold positioned on the negative side of the threshold distribution of memory cells is to be read, the data is read by applying a higher voltage to a source than to a gate.